ECR apparatus with magnetic coil for plasma refractive index control

ABSTRACT

The present invention describes a technique to control the radial profile of microwave power in an ECR plasma discharge. In order to provide for a uniform plasma density to a specimen, uniform energy absorption by the plasma is desired. By controlling the radial profile of the microwave power transmitted through the microwave window of a reactor, the profile of the transmitted energy to the plasma can be controlled in order to have uniform energy absorption by the plasma. An advantage of controlling the profile using the window transmission characteristics is that variations to the radial profile of microwave power can be made without changing the microwave coupler or reactor design.

The United States Government has rights in this invention pursuant toContract No. DE-AC05-840R21400 and ERD-89-876 between the Department ofEnergy and SEMATECH, INC.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of plasma processing and,more particularly, to the use of plasma equipment for modification ofmaterials.

2. Prior Art

Plasma processing equipment is used extensively in the industry for theprocess modification of materials. These modifications include etchingand depositing of films for the fabrication of microelectronic circuitsand semiconductor devices. The modifications may also includeimplantation of chemical species that change the friction and wearproperties of surfaces.

A plasma is a gas (or a gas mixture) which is energized so that it ispartially decomposed into species that are electrically charged.Although a number of techniques are known for energizing the gas,electron cyclotron resonance (ECR) plasma processing is one knowntechnique for controlling the plasma with the use of electric (E) andmagnetic (B) fields.

ECR plasma production involves the interaction of microwave power with amagnetized plasma at a particular resonant frequency. A well-known andpopularly used ECR practice is based on resonance values given by thecold plasma theory. The magnetic field for this resonance isproportional to the microwave frequency and has a magnetic value of 875Gauss (G) at the resonant microwave frequency of 2.45 gigahertz (GHz).The prior art abounds with references pertaining to the production ofplasma at the 875 G/2.45 GHz ECR resonance. Two descriptive articles onECR plasma production are 1) "Electron cyclotron resonance microwavedischarges for etching and thin-film deposition", Jes Asmussen; J.Vac.Sci. Technol. A, Vol. 7, No. 3; May/June 1989; pages 883-893; and 2)"Microwave plasma: its characteristics and applications in thin filmtechnology"; J. Musil; Vacuum, Vol. 36, Nos. 1-3; 1986; pages 161-169.

The plasma technology has been utilized in the semiconductor industryfor the purpose of processing the surface of a semiconductor substrate,such as a silicon wafer. Typically, the plasma is used to deposit alayer onto a wafer or, alternatively, it is used to etch a layer on thewafer. A variety of plasma reactors are known in the prior art and thesemiconductor industry has developed many enhancements to these reactordesigns.

For example, U.S. Pat. No. 4,101,411 discloses an apparatus wherein thesurface of a substrate is etched by using ions in a generated plasma.U.S. Pat. No. 4,298,419 describes an apparatus which has a plasmagenerating chamber for the generation of the plasma and a separateetching chamber where the substrate sample is processed. Subsequentadaptation of the ECR technology was then utilized to practice thetechnology described in 3) "CVD utilizing ECR plasma"; Transactions of31st Semiconductor Integrated Circuit Technology Symposium; Dec. 3-4,1986; pages 49-54; which is shown as prior art in FIG. 1 of U.S. Pat.No. 4,876,983.

However, it soon became apparent that the long transport distance fromthe plasma formation region to the specimen resulted in the loss(sometimes substantial loss) of the plasma energy, tending to result inthe density of the plasma reaching the specimen to be lesser than thatof the plasma at or near the resonance zone. Furthermore, the divergenceof the magnetic flux lines in the radial direction tended to cause theplasma to diverge as it is transported to the wafer, also resulting inreduced plasma density at the wafer relative to the resonance zone. Thistechnique appears to work reasonably well for processes which are notion dominated (that is, high plasma densities are not required at thewafer), which do not need low ion energies, and/or which do not requirerapid processing.

Although the earlier reactor technology provided desired results forsemiconductor wafer processing in the past when wafer diameters were inthe order of 4-5 inches, these prior art techniques appear to beinadequate for the processing of larger diameter wafers. As siliconsemiconductor technology advances to manufacture devices on wafershaving submicron dimensions of 0.5 micron and below, such as 0.35, 0.25,0.15 micron and below, the industry has moved toward single waferprocessing utilizing larger diameter wafers. It has been shownexperimentally that as wafer size increases, at and beyond diameters of200 mm (8 inch), and where topographic features on the wafer are madeever smaller, uniformity is a critical constraint in performing ECRprocessing.

A variety of techniques have been tried to attempt to provide a moreuniform processing over the complete surface of the wafer. One techniqueis described in U.S. Pat. No. 4,876,983 in which the formation of theplasma (in essence the location of the resonance) is moved closer to thespecimen. This is achieved by the ECR zone being located at leastpartially within the specimen chamber and nearer to the wafer.

A related technique is described, in an article entitled 4) "Extremelyhigh selective, highly anisotropic, and high rate electron cyclotronresonance plasma etching for n+ poly-Si at the electron cyclotronresonance position"; Samukawa et al; J. Vac. Sci. Technol. B, Vol. 8,No. 6, Nov./Dec. 1990; pages 1192-1198; which suggested that a sourcewhich would allow processing at higher pressures (over 2 to 3 mTorr)within a few (2 to 10) cm of the ECR zone would reduce density gradientsand thus ion energies. In order to preserve uniformity, the statedcriterion was that the ECR zone of 875 gauss (G) should have flatcontour in space. That is, the suggested technique was to have a flatresonance zone in the reactor and the divergence of the field was to bereduced to maintain the highest possible density with minimum ionenergy. The divergent magnetic field is corrected and collimated by theuse of a submagnetic field.

However, having a substantially flat magnetic field contour in itselfdoes not assure the formation of a plasma having uniform density acrossthe complete surface of the wafer. That is, for example, uniformmicrowave power density must be transmitted to the location of theresonance for the formation of a plasma having uniform density.

Experimentations have shown that some plasma control can be obtained bythe use of multipole surface magnetic fields to confine the ECRdischarge. U.S. Pat. Nos. 4,483,737 and 4,745,337 describe the use ofthis technique. A multipole design for maintaining a low magnetic fieldand good uniformity is disclosed in U.S. Pat. No. 5,032,202.

Some additional control is possible by using microwave couplers withdifferent radial profiles of microwave power which is incident on themicrowave window as determined by the waveguide mode of the microwavecoupler. An example of such an approach is the use of the TM₀₁ modeinstead of the more typical TE₀₁ rectangular or TE₁₁ circular modes.However only limited control is possible as the laws of physicsconstrain the number of possible modes for the dimensions andfrequencies of typical processing systems.

Although fairly uniform ECR discharges can be obtained with the aboveprior art technology, uniform plasma discharge to the wafer is difficultto achieve especially for submicron device fabrication on largerdiameter wafers.

SUMMARY OF THE INVENTION

The present invention describes a technique to control the radialprofile of microwave power in an ECR plasma discharge. In order toprovide for a uniform plasma density at the wafer, uniform energyabsorption by the plasma at the ECR zone is desired. By controlling thefraction of the incident microwave power on the microwave window of areactor which is transmitted to the plasma at any particular radialposition on the window, significantly more uniform energy absorption bythe plasma can be produced.

The transmission of microwave power through a window is determined bythe index of refraction of the window, the thickness of the window, andthe index of refraction of the medium (in this case the process plasma)in contact with the window. The present invention is based on varyingthese three factors, either singly or in combination, across the radiusof the microwave window to control the fraction of incident microwavepower which is transmitted through the window into the plasma.

In one technique, the radial profile of the plasma index of refractionat the window is controlled by changing the magnetic field radialprofile by the use of magnets having small quadrapole trim coils. Smallvariations in the magnetic field profile at the window producesubstantial variations in the index of refraction of the plasma, whichcan provide for more power to be absorbed by the plasma locatedunderlying the periphery of the window, than at its center. Thispartially compensates for the decrease in microwave electric fieldamplitude at the periphery thereby producing a more uniform plasmaformation at the ECR zone.

In a second technique, the microwave window is altered to have aradially varying window thickness and/or index of refraction in order toattenuate (transmit less) and/or reflect microwave energy at the windowcenter. The profile of the incident microwave energy transmitted throughthe window is altered at the window to provide for a more uniformtransfer of microwave power to the plasma.

Both of these techniques either alone or together provide for a way tocontrol a radial profile of the microwave power being transmitted andabsorbed by the plasma.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an ECR plasma reactor.

FIG. 2 is a graphical representation showing the relative electric fieldmagnitude versus normalized radius for TM₀₁ and TE₁₁ modes of operation.

FIG. 3 is a graphical representation of the magnitude of electric fieldversus normalized radius for a polarized component of TE₁₁ mode.

FIG. 4 is a graphical representation of the magnitude of circularlypolarized power versus normalized radius for the TE₁₁ mode.

FIG. 5 illustrates the magnitude of the electric field of an incidentmicrowave reaching a microwave window of the reactor.

FIG. 6 shows the conversion of the incident microwave E(r) to thetransmitted microwave T(r) due to the presence of the window.

FIG. 7 illustrates a window of the present invention having acurvilinear surface to alter the radial profile of the incidentmicrowave.

FIG. 8 illustrates a window of the present invention having a steppedsurface to alter the radial profile of the incident microwave.

FIG. 9 illustrates a window of the present invention having a dielectricgradient surface to alter the radial profile of the incident microwave.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A technique for controlling the radial profile of microwave power in anECR plasma discharge is described. In the following description,numerous specific details are set forth, such as specific devices,measuring techniques, reactor parameters, etc., in order to provide athorough understanding of the present invention. However, it will beobvious to one skilled in the art that the present invention may bepracticed without these specific details. In other instances, well knownprocesses and structures have not been described in detail in order notto unnecessarily obscure the present invention. In addition, asimplified description of the process by which microwave power interactswith the microwave window and the process plasma is used to illustratethe underlying principles of the invention.

As was described in the description of the various prior art techniques,a number of approaches have been devised to process a wafer utilizingelectron cyclotron resonance (ECR) produced plasma. In order touniformly process the surface of a wafer, it is imperative to obtainuniform plasma density at the surface of the wafer. Without the presenceof a uniform plasma density at the surface of the wafer, etching and/ordeposition steps performed on the wafer will typically result in theformation of non-uniform layers on the wafer. Such non-uniformity iscritical when device dimensions are in the submicron range. In addition,uniformity problems are compounded since such submicron dimensioneddevices are now being formed on larger diameter wafers (such as 200 mmdiameter or greater wafers) which are now being utilized for themanufacture of silicon devices.

The plasma must now be uniform over the increased surface area of thelarger diameter wafer. In order to obtain plasma uniformity at thesurface of the wafer, a number of factors must be controlled.Experimental results have shown that plasma density (or approximatelyequivalent, ion current densities or processing rates) are proportionalto the applied microwave power. Thus plasma uniformity, in addition tothe losses between the ECR zone and the wafer, is also dependent on theuniformity of the absorbed microwave power at the ECR zone. Thisdependence on uniformity of absorbed microwave power is especially truefor ECR sources where the magnetic field is sufficiently strong toprevent radial transport from smoothing gradients in plasma density.

The absorbed microwave power in turn depends on the magnitude of theincident microwave electric field (actually the square of the field) andon the fractional amount of that power which is transmitted to theplasma where it is subsequently absorbed. Although prior art techniqueshave addressed these concerns indirectly in order to provide foruniformity, they have not addressed control of the incident microwavepower profile at the ECR zone to control uniformity.

In order to control plasma density uniformity at the wafer, the radiallydependent convolution of three factors must be controlled. First is themagnitude of the incident microwave power which is favorable polarized.The second factor is the amount of that incident power which istransmitted into and subsequently absorbed by the plasma. Lastly, thelosses (typically pressure induced) between the ECR zone and the wafermust be controlled.

A number of various prior art techniques can be readily used to controlthe third factor. One such technique is reducing the physical spacingbetween the ECR zone and the wafer. Another technique is the use oflower pressures.

A technique to influence the first factor is described in 5) "Optimizedmicrowave coupling in an electron cyclotron resonance etch tool";Stevens et al." J. Vac. Sci. Technol. A, Vol. 9, No. 3; May/June 1991;pages 696-701. In this technique, globally improved coupling of the 2.45GHz microwave is obtained by using a right-hand circularly polarized(r.h.c.p.) mode which is matched to the plasma impedance through aquarter-wave transformer.

The present invention provides for a technique for the control of thesecond factor, which is to control the fraction of the incident powertransmitted through the window which is subsequently absorbed by theplasma to produce a uniformly dense plasma at the ECR zone.

Referring to FIG. 1, an ECR plasma reactor 9 is shown. The basic designof reactor 9 can be of a variety of prior art ECR plasma reactors, whichtypically includes a chamber 10 coupled to a microwave transmissionmedium, such as a waveguide 11. The waveguide 11 may include a couplingmechanism, such as a microwave coupler, to couple the waveguide 11 tothe chamber 10. (For simplicity, waveguide 11 is used to designateeither a waveguide or a waveguide/coupler in the followingdescriptions). At the reactor end of waveguide 11 is a microwave window12 for coupling the microwave energy into reactor 10. A specimen 13,which the plasma is to operate upon, resides on a platen, such as achuck 14. For example, specimen 13 can be a silicon semiconductor wafer.Magnets 15 are placed around the reactor chamber 10 for providing thenecessary magnetic field (B) for ECR operation. It is to be noted thatalthough only circular magnet 15 is shown, the number, placement andtype are strictly a design choice and most ECR magnet(s) used in theprior art can be readily used. When activated an ECR zone 18 is formednear to the upper surface of the specimen 13. These aspects of theinvention are known in the art and are adaptable for use with thepresent invention.

Like light incident on a dielectric, some fraction of an incidentmicrowave beam travelling in waveguide 11 can be reflected from and/ortransmitted through window 12. This fraction of the instant beam beingreflected and/or transmitted at window 12 can vary across the width ofwindow 12. Furthermore, the incident microwaves also have a profile,depending on the source and waveguide 11.

Referring to FIG. 2, a variation in magnitude of the Electric field (E)for two microwave circular waveguide modes commonly used for plasmaproduction is shown as an example. Field patterns for the TM₀₁ and TE₁₁modes 20 and 21, respectively, are shown comparing the magnitudes to anormalized radius. The TM₀₁ mode is intrinsically symmetric, (that is,no variation in angle within the circular wave guide) while the TE₁₁mode may be polarized in any direction. The particular choice in FIG. 2,pattern 21 is the sum of two TE₁₁ modes which are spatially rotated 90degrees from each other, for example "x" and "y," and are alsotemporally separated 90 degrees in phase (one quarter of the period ofthe microwaves). This produces a mode which is substantially circularlypolarized.

It is to be noted that both modes have a substantial variation inradius. The TE₁₁ mode is preferable since it has less variations inmagnitude across its radius. However, even for the TE₁₁ mode thesituation is actually worse than illustrated since the fraction of themicrowave E-field that is favorably polarized is highest in the centerand falls toward zero at the edge of the window 12. This polarizationloss is better illustrated by pattern 22 in FIG. 3. In FIG. 3, pattern22 shows the magnitude of the polarized component of the electric fieldversus normalized radius.

Referring to FIG. 4, pattern 23 shows the magnitude of power (P) of thepolarized component versus the normalized radius. The polarization lossis of a significant concern since only the favorably polarized portionis available for high density plasma production. Worse yet, the usefulincident power is actually the square of the magnitude of the polarizedcomponent of the electric field, as is noted in FIG. 4. Thus, it isreadily apparent why typically only a fraction of the window area can beutilized for processing. If a typical prior art window is used forwindow 12, the outer periphery of the window cannot be utilizedeffectively if uniform plasma discharge is desired. For example, an 8.75inch wide window is needed in order to process a 6 inch (150 mm)diameter wafer. For 200 mm wafer processing, generally a larger windowis required. Accordingly, even though fairly uniform plasma densitiesmay be achieved, the size of the window needed to obtain such resultsplaces a significant disadvantage in designing a reactor.

Referring to FIG. 5, it shows the profile of the magnitude of theE-field incident on window 12. The incident E-field is of TE₁₁ mode sothat the maximum magnitude is at the center of the window 12 anddecreases toward the periphery of window 12.

Referring to FIG. 6, it shows the window 12 as well as the transmissioncharacteristics of the incident E field. E(r) is the incident electricfield, Ew(r) is the incident component through the window and T(r) isthe electric field transmitted to the reactor and, hence, to the plasma.The thickness of the window is noted by "d". Locally, at the interfacebetween the window and the plasma, the incident microwave experiences aproblem similar to a dielectric slab between two different half spaces.The underlying principles which govern the microwave reflection andtransmission are of concern.

With respect to the flow of power from the window to the plasma, thepower P is determined by,

    P=4n.sub.p /(n.sub.p +n.sub.w).sup.2                       (Equation 1)

where n_(w) is the index of refraction for the window and n_(p) is theindex of refraction for the plasma.

The plasma index of refraction (n_(p)) is determined by,

    n.sub.p =sqrt(1+(n/n.sub.crit)/(B/B.sub.0 -1))             (Equation 2)

where B/B₀ is the ratio of the local magnitude of the magnetic field atthe window to 875 gauss and n/n_(crit) is the ratio of the local plasmadensity to the so called critical density (n_(crit) is approximately8×10¹⁰ cm⁻³ for a microwave frequency of 2.45 GHz). For typicalmaterials, n_(w) is in the range of 2 to 3. For typical plasmaparameters, n_(p) is in the range of 7 to 15.

Because n_(p) is almost always larger than n_(w), and because of therelationship in Equation 1, it follows that lower values of n_(p) willresult in a larger fraction of the power being locally transmittedthrough the window. Since the magnetic field near the window has anatural increase from the center of the window to the periphery, n_(p)will decrease in value from the center of the window to the periphery.This follows from the above expression for n_(p) as the quantity B/B₀ -1increases. Note that since B/B₀ is approximately near unity (typically1.1 or 1.2 at the window center), even small variations in magneticfield profile at the window can produce substantial variations in n_(p).As a result more power will be absorbed by the plasma located underlyingthe periphery of the window 12 than at its center. This increased powerabsorption by the plasma located at the periphery of the ECR zone atleast partially compensates for the decrease in microwave E fieldamplitude at the window 12 and produces a more uniform plasma at the ECRzone. Thus, a change in the radial profile of the magnetic field at thewindow allows for a technique to control the power to the plasma forplasma uniformity.

In order to control the radial profile of the magnetic field at thewindow to achieve plasma uniformity, the present invention uses a systemof coils to control the B field at the window. This can be achieved bythe initial choice of the size, placement, and current levels of themain magnets 15, as well as by the use of trim coils. For example, smallquadrapole trim coils 25 and 26 which produce no net change in the fieldin the axial direction, can be used to change the radial profile. Thesecoils 25 and 26 are placed typically around the window 12, as is shownin FIG. 1. It is obvious to an experienced practitioner in the design ofmagnetic fields that a wide variety of electromagnet coil positions andcurrent distributions, as well as permanent magnets, may be used toachieve the desired radial variation in magnetic field. These types ofchanges in the magnetic field structure may be constrained as theposition and shape of the ECR zone 18 could affect losses, however, somesignificant measure of independent control is possible with sufficientlywell localized magnetic field perturbations.

Another technique for controlling the radial profile of the incidentmicrowave in an ECR discharge, is the microwave window itself. As wasshown in FIG. 5, the magnitude of the E-field of the microwave isgreater at the center of the window than at its periphery for TE₁₁ mode.By controlling the attenuation and/or the reflection at the window, theradial profile of the transmitted power T(r) can be controlled at window12. The properties of the reflected and transmitted wave from adielectric slab have been used for optical coatings on lenses and a hostof other applications in the prior art. These techniques can be readilyadapted to provide a microwave window having an effective thickness(effective is defined as thickness measured in wavelengths) and index ofrefraction. The nature of these techniques is pointed to by the analogto Equation 1 with a dielectric slab instead of a simple interface.

When the second interface is included in the physics analysis, Equation1 becomes more complicated and now depends on the thickness of thewindow, d.

Namely: ##EQU1## where k=2π/λ and λ is the microwave wavelength in thewindow. It should be noted that this discussion is based on a simplifiedmodel. However, the more accurate models, which in general requirecomputer-based analysis, have the same characteristics and can bereadily determined for a given design.

Referring to FIG. 7, a microwave window 12a is shown having its one faceadjacent to the reactor 10 substantially flat, while its opposite facenear the waveguide 11 has a convex surface to provide a curvilinearsurface. The thickness of the window at the periphery is chosen formaximum transmitted microwave power (often it will be for a window onequarter wavelength in thickness) while the dimension in the middle isthicker than needed for optimum coupling. Alternatively, but notillustrated, the window could be made thinner in the middle withequivalent results. Also not shown, the direction of curvature could betowards the plasma.

Referring to FIG. 8, a differently structured microwave window 12b isshown. Window 12b has an equivalent effect as window 12a, except that itis stepped instead of being continuously curvilinear. The window 12b iscomprised of a number of stepped sections atop a base section. The basesection is disposed completely across the opening and each subsequentsection is of a smaller width. Essentially, FIG. 8 provides for adiscrete surface while FIG. 7 provides for a curvilinear surface. Thewindows 12a and 12b provide for a varying d depending on the radiallocation. As was the case with the window 12a of FIG. 7, the shape couldbe altered to provide equivalent results.

Alternatively, a window having a constant physical thickness can be usedas is shown in FIG. 9. Window 12c has a constant dimension, but has adielectric gradient which is shown by shaded region 30. The dielectricgradient effectively operates to provide equivalent results as windows12a.

Windows 12a-12c provide for radially varying attenuation to control themicrowave power profile transmitted to the plasma. Windows 12a-12cattenuate (transmit less) the portion of the TE₁₁ microwave near thecenter of the respective window where the magnitude is greater. By thisattenuation, power transmitted to the plasma is attenuated near thecenter of the ECR zone 18 and the plasma made more uniform.

It is to be appreciated that a variety of techniques can be utilized tocontrol the window thickness and/or the index of refraction to obtaindesired E-field profile to be transmitted to the plasma. Typically, thedesired profile is a T(r) having substantially uniform magnitude.

It is to be noted that the desired T(r) to obtain uniform plasma densitycan be achieved with using either the magnetic control or the windowdesign. Furthermore, both techniques can be used together. In such acombination, the window will provide for a fixed means for coarsecontrol of the radial profile and the trim coils would provide for avariable means for fine tuning the radial profile. Such fine tuningcontrols may be needed in order to compensate for variations in plasmaimpedance and dielectric loss between the ECR zone 18 and specimen 13.

Additionally, polarization of the incident microwave is of littleconcern when practicing the present invention. The non-polarizedcomponent can be matched or, if not matched, then it can be reflected atthe window. Thus, the mode of operation is inconsequential for an ECRplasma system using the present invention.

Finally, it is to be noted that the resonance can be selected at adesired zone. Although 2.45 GHz/875 G values are derived from the coldplasma theory, investigations in this area indicate that the morerealistic ECR zone is established at 2.45 GHz/925-975 G. The phenomenonis speculated to be caused by the doppler shifting of the microwaveacting on the electrons of the plasma. However, the location of theactual ECR zone is tangential to the practice of the present invention.

Thus, the present invention provides for a technique to control theradial profile of the microwave power at the window in order to controlECR plasma uniformity without changing the coupler or the reactordesign.

I claim:
 1. An electron cyclotron resonance (ECR) plasma apparatus forprocessing a specimen comprising:a plasma reactor for having saidspecimen disposed therein and in which an ECR zone is formed fordischarge of plasma to process said specimen; an electrical energysource for providing microwave energy to generate an electric fieldcomponent for formation of said ECR zone in said reactor; a microwavewindow coupled between said plasma reactor and said electrical energysource for directing said microwave energy into said reactor; at leastone main magnetic coil coupled about said reactor for generating amagnetic field component for formation of said ECR zone in said reactor;a secondary magnetic coil coupled about said reactor for generating asecondary magnetic field for controlling the radial profile of saidmagnetic field component at said microwave window in order to alter theindex of refraction of said plasma, wherein providing for asubstantially uniform power absorption across said ECR zone to provideuniform density plasma for discharge to said specimen.
 2. The ECR plasmaapparatus of claim 1 wherein said secondary magnetic coil is disposedproximally about said microwave window.
 3. The ECR plasma apparatus ofclaim 2 wherein said secondary magnetic coil is actually comprised of aplurality of coils to provide quadrapole trim coils for controlling saidradial profile, but producing appreciably no net change in radialdirection of said magnetic field component.
 4. In an electron cyclotronresonance (ECR) plasma processing apparatus, having a reactor forprocessing a gas, wherein said reactor is coupled to an electricalenergy source which provides microwave energy and wherein said reactoris also coupled to a magnetic coil, a method for providing uniformdensity in a plasma discharge, comprising the steps of:introducing saidgas into said reactor; introducing microwave energy through a microwavewindow into said reactor in order to generate an electric fieldcomponent for formation of an ECR zone in said reactor; activating saidmagnetic coil to introduce a magnetic field component for formation ofsaid ECR zone in said reactor; controlling a radial profile of saidmagnetic field component at said microwave window in order to vary theindex of refraction of said plasma to provide for a substantiallyuniform power absorption across said ECR zone to provide uniform densityplasma for discharge to a target.
 5. The method of claim 4 wherein saidstep of controlling said radial profile of said magnetic field componentis performed by a secondary magnetic coil coupled about said microwavewindow.
 6. The method of claim 4 wherein said step of controlling saidradial profile of said magnetic field component is performed by aplurality of secondary magnetic coils coupled about said microwavewindow to provide quadrapole trim coils for controlling said radialprofile, but producing appreciably no net change in radial direction ofsaid magnetic field component.